VOLUME 9, ISSUE 1, ARTICLE 8 Knight, E. C., N. A. Mahony, and D. J. Green. 2014. Crop type influences edge effects on the reproduction of songbirds in sagebrush habitat near agriculture. Avian Conservation and Ecology 9(1): 8. http://dx.doi.org/10.5751/ACE-00662-090108 Copyright © 2014 by the author(s). Published here under license by the Resilience Alliance.
Research Paper Crop type influences edge effects on the reproduction of songbirds in sagebrush habitat near agriculture Elly C. Knight 1, Nancy A. Mahony 2 and David J. Green 1 1Simon Fraser University, 2Environment Canada
ABSTRACT. Extensive fragmentation of the sagebrush shrubsteppe of western North America could be contributing to observed population declines of songbirds in sagebrush habitat. We examined whether habitat fragmentation impacts the reproduction of songbirds in sagebrush edge habitat near agriculture, and if potential impacts vary depending on the adjacent crop type. Specifically, we evaluated whether nest abundance and nest survival varied between orchard edge habitat, vineyard edge habitat, and interior habitat. We then examined whether the local nest predator community and vegetation could explain the differences detected. We detected fewer nests in edge than interior habitat. Nest abundance per songbird was also lower in edge than interior habitat, although only adjacent to vineyards. Nest predation was more frequent in orchard edge habitat than vineyard edge or interior habitat. Predators identified with nest cameras were primarily snakes, however, reduced nest survival in orchard edge habitat was not explained by differences in the abundance of snakes or any other predator species identified. Information theoretic analysis of daily survival rates showed that greater study plot shrub cover and lower grass height at nests were partially responsible for the lower rate of predation-specific daily nest survival rate (PDSR) observed in orchard edge habitat, but additional factors are likely important. Results of this study suggest that different crop types have different edge effects on songbirds nesting in sagebrush shrubsteppe, and that these reproductive edge effects may contribute to observed declines of these species. Habitat managers should avoid the creation of new orchard-sagebrush habitat edges to avoid further impacts on already declining songbird populations.
Effet de bordure sur la reproduction de passereaux dans des milieux d’armoises variable selon le type de culture adjacente RÉSUMÉ. La fragmentation à grande échelle de la steppe arbustive à armoises de l’ouest de l’Amérique du Nord contribue peut-être aux déclins observés des passereaux des milieux d’armoises. Nous avons examiné si la fragmentation avait un impact sur la reproduction des passereaux dans des milieux d’armoises en bordure de milieux agricoles, et si les effets potentiels variaient selon le type de culture adjacente. Plus particulièrement, nous avons évalué si le nombre de nids et leur survie différaient entre les milieux de bordure de vergers, les milieux de bordure de vignobles et les milieux d’intérieur. Nous avons ensuite examiné si la communauté locale de prédateurs de nids et la végétation pouvaient expliquer les différences détectées. Nous avons trouvé moins de nids dans les milieux de bordure que dans les milieux d’intérieur. Le nombre de nids par espèce était également plus faible dans les bordures par rapport aux milieux d’intérieur, mais ce, seulement près des vignobles. La prédation des nids était plus fréquente dans les bordures de vergers que dans les bordures de vignobles ou les milieux d’intérieur. Les prédateurs identifiés par caméra au-dessus des nids étaient surtout des serpents. Toutefois, la survie plus faible des nids dans les bordures de vergers n’a pas pu être expliquée à partir des différences du nombre de serpents ou de tout autre prédateur identifié. L’analyse par théorie de l’information des taux de survie quotidiens a montré que le couvert arbustif plus élevé dans nos parcelles-échantillons et la plus petite taille des herbacées aux nids étaient en partie responsables du taux moins élevé de survie quotidien des nids spécifique à la prédation observée dans les bordures de vergers, mais d’autres facteurs importants agissent vraisemblablement. Les résultats de notre étude montrent que les différents types de culture ont des effets de bordure variés sur les passereaux nichant dans la steppe arbustive à armoises, et que ces effets de bordure sur la reproduction participent peut- être aux déclins observés de ces espèces. Les gestionnaires d’habitat devraient éviter de créer de nouveaux milieux de bordure vergers- armoises afin de prévenir davantage d’effets négatifs sur des populations de passereaux déjà en déclin. Key Words: agriculture; conservation; edge effects; fragmentation; nest survival; sagebrush; songbird
INTRODUCTION temperate breeding birds, and several types of edge effects have Anthropogenic habitat fragmentation continues to increase the been detected. The presence of edges can influence habitat choice, proportion of edge habitat in terrestrial landscapes. When edge with specialist species often less abundant in edge habitat than habitat is created, the ecological communities present are exposed away from edges (Ewers and Didham 2006). The individuals that to different biotic and abiotic processes (Ewers and Didham 2006, reside nearer to edges can have lower pairing success (Van Horn Fischer and Lindenmayer 2007). The impact of these processes, et al. 1995, Bayne and Hobson 2001), or suffer higher nest or “edge effects” (Leopold 1933) has been studied extensively in predation (reviewed by Lahti 2001, Batáry and Báldi 2004,
Address of Correspondent: Elly C. Knight, Department of Biological Sciences, 8888 University Drive, Burnaby, BC V5A 1S6, Canada, ecknight@sfu. ca Avian Conservation and Ecology 9(1): 8 http://www.ace-eco.org/vol9/iss1/art8/
Stephens et al. 2004). Most of the early edge effect studies adjacent agriculture, orchard, or vineyard. First, we evaluated indicated nest predation is consistently higher near habitat edges whether songbirds in sagebrush habitat nested more frequently (Andrén et al. 1985, Paton 1994, Major and Kendal 1996). in edge or interior sagebrush habitat. We then tested whether nest However, recent studies suggest that an edge effect on nest predation was more frequent in edge habitat than interior habitat. predation is often not detected and can vary with many factors We used remote cameras placed at natural nests in the sagebrush (Hartley and Hunter 1998, Sisk and Battin 2002, Batáry and Báldi shrubsteppe to identify nest predator species and used this 2004). For example, edge effects appear to be more prominent in information to evaluate whether differences in nest predation rate landscapes with high levels of anthropogenic fragmentation could be attributed to the abundance of known nest predators. (Donovan et al. 1997, Lahti 2001). Given the number of factors that can influence edge effects, reviews suggest potential edge METHODS effects on nest predation should be studied on a regional basis (Sisk and Battin 2002, Stephens et al. 2004). Study area We studied songbirds breeding in sagebrush shrubsteppe habitat The most commonly proposed explanation for an edge effect on in the Okanagan region of British Columbia, Canada and nest predation is increased abundance of nest predators in edge Washington, USA from May to August 2011 and 2012 (Fig. 1A). habitat (Chalfoun et al. 2002, Lahti 2009, Spanhove et al. 2009). Sagebrush shrubsteppe habitat is characterized by big sagebrush However, without identification and quantification of the (Artemisia tridentata) and bunch grasses. The Okanagan is the abundance of the dominant nest predator, this explanation northernmost region of the shrubsteppe that extends across the remains speculative. Furthermore, it is difficult to understand why intermountain west of North America. Human activities exert predators might be more abundant in edge habitat without also high pressure on the sagebrush habitat in the Okanagan and understanding the life history of the dominant nest predator. approximately 35% has been converted to other land uses such as Studies have traditionally ignored predator identity because it was orchards and ground crops (USA: 38% Dobler et al. 1996, difficult to record nest predation events (Lahti 2009). Recent Canada: 33% Iverson et al. 2008). Currently, both sagebrush studies that use modern camera technology to identify nest habitat and orchard areas are being converted to vineyards to predators have had more success explaining the presence or meet increasing consumer demand for wine. absence of edge effects on nest predation. For example, an overall absence of an edge effect on nest predation can occur because predation by one species in edge habitat is offset by that of another Fig. 1. A. Sites for studying the songbird community in species in interior habitat (Benson et al. 2010, Cox et al. 2012). sagebrush shrubsteppe habitat in the Okanagan region of British Columbia, Canada, and Washington, USA. B. Study Conversion of land for agriculture is a leading cause of global site schematic overlaid on imagery of a real study site. Each habitat fragmentation (Foley et al. 2005). An edge effect on nest study site consisted of a pair of study plots: one plot adjacent predation is more often, although not always, detected near to agriculture (edge) and one away from agriculture (interior). agricultural edges than other edge types (Andrén 1995, but see Davis et al. 2006). Additionally, predators are often more abundant in edge habitat near agriculture than other land uses or interior habitat (Chalfoun et al. 2002). Agricultural edge habitat may have high nest predation rates because predators are attracted to supplemental food sources or vegetation structure within the adjacent crop (Chalfoun et al. 2002). Thus, the strength of edge effects should also vary with crop type because both food source and vegetation structure vary widely among crops. Some studies and reviews to date may have failed to detect edge effects on nest predation because they have generalized across crop types (e.g., Lahti 2001). Extensive habitat fragmentation by agriculture has contributed to the imperilment of sagebrush shrubsteppe habitats in North America. Many of the species that inhabit the sagebrush shrubsteppe are also imperilled and show apparent population declines (Knick et al. 2003, Wisdom et al. 2003, Sauer et al. 2014). Habitat fragmentation is considered a contributing factor to these declines because predation rates of songbird nests are generally higher in shrubsteppe landscapes fragmented by agriculture than Study species unfragmented landscapes (Vander Haegen et al. 2002, Vander The suite of songbirds that nest in the Okanagan sagebrush Haegen 2007). This landscape level pattern may arise because of shrubsteppe include Brewer’s Sparrows (Spizella breweri), Lark edge effects on nest predation in fragmented landscapes (Bender Sparrows (Chondestes grammacus), Vesper Sparrows (Pooecetes et al. 1998, Ries et al. 2004, Fletcher et al. 2007). gramineus), and Western Meadowlarks (Sturnella neglecta). In this study, we examined agricultural edge effects on the Population trends for these species in the Great Basin bird breeding success of four species of songbirds in sagebrush habitat, conservation region are -0.8%, -1.0%, -0.8%, -1.1% change per and investigated whether edge effects varied with crop type of the year, respectively (Sauer et al. 2014). The last three species are all Avian Conservation and Ecology 9(1): 8 http://www.ace-eco.org/vol9/iss1/art8/
open-cup ground nesters that conceal their nests at the base of days following successful deployment. We reviewed the nest bunchgrasses, small shrubs, or large forbs (Pitkin and Quattrini camera footage to confirm the fate of all nests with cameras and 2010). In contrast, Brewer’s Sparrows place their open-cup nests identify the species of nest predator responsible for any nest off the ground in big sagebrush (Rotenberry et al. 1999). predation events. Study plots Nest predator abundance We selected study sites in patches of sagebrush shrubsteppe that We measured the abundance or presence/absence of potential were adjacent to agriculture, that were large enough to include predator species of songbird nests at each plot in the sagebrush interior habitat at least 400 m from agriculture in all directions, shrubsteppe. Relative abundance of small mammals at each plot that had similar vegetation across the study site, and where we was measured with track tubes, which collect footprints using a were given permission to access the property. In total, breeding food bait (adapted from Mabee 1998, Glennon et al. 2002, Wiewel of songbirds was monitored at 18 sites, each consisting of a pair et al. 2007). Two replicate sets of track tubes were set out at each of 160 m by 100 m study plots (Fig. 1A, 1B). Plot size was chosen plot with 36 days between the two sets. There were 24 tubes spaced to ensure observers would be able to adequately survey songbirds, 30 m apart from each other on an 8 x 4 grid in each replicate set. predators, and vegetation at each plot. Each pair consisted of one Track tubes were 30 cm lengths of vinyl white downspout with plot in sagebrush shrubsteppe adjacent to agriculture (“edge felt pads glued at either entrance, and a length of clear adhesive habitat”) and one plot 400 - 700 m into shrubsteppe habitat away drawer liner to collect prints. Each felt pad was saturated with a from anthropogenic land uses (“interior habitat”). Half of the mixture of mineral oil and carbon black powder. Tubes were plots were surveyed in 2011, and the remainder in 2012. Ten edge baited with a small amount of peanut butter on the inner ceiling plots were situated adjacent to orchards, and eight adjacent to of the tube and set under a shrub. The tubes were then left at each vineyards to test for different edge effects among agricultural study plot for four days to collect prints. All prints were identified types (Fig. 1A). as mouse (Superfamily Muroidea) because faint or overlapping prints were often indistinguishable to species. An abundance index Nest surveys was calculated for each replicate set at each plot by dividing the Plots were searched for songbird nests with a systematic, constant number of track tubes set out by the number of tubes that effort strategy to avoid bias at any given plot. Each plot was collected prints. The abundance indices from the two replicate sets searched for two hours every four days using a combination of of track tubes were highly correlated (r = 0.767, P < 0.001), so systematic and observational techniques. Systematic nest searches we averaged the two replicate sets of track tubes at each plot to were conducted by walking transects while waving a wooden rod calculate the abundance index for each plot. to flush birds. Observational searches using behavioral cues of Presence or absence of medium and large sized mammals (black adult birds were conducted on all study species individuals for bear [Ursus americanus], coyote [Canis latrans], domestic cat [Felis which the study plot made up at least a portion of their territory catus], domestic cow [Bos primigenius], long-tailed weasel to maximize the number of nests found. Upon discovery, the age [Mustela frenata], raccoon [Procyon lotor], yellow-pine chipmunk of the contents of each nest was estimated by candling eggs [Noetamias amoenus]) at each plot was estimated using a (Lokemoen and Koford 1996), or aging nestlings based on combination of incidental observations and track stations published descriptions (Dawson and Evans 1960, Baepler 1968, (adapted from Kuehl and Clark 2002). Incidental observations of Rotenberry et al. 1999, Davis and Lanyon 2008). We confirmed scat, dens, or live individuals were recorded from 1 May 1 to 31 that observers were able to candle incubated eggs to within two July. Equal time was spent at each plot by each observer to avoid days of the correct age using information on mean incubation any observer or site bias in detection (35.8 ± 1.8 hours per plot). period and the hatch days of aged nests. Each nest found was In addition, four track stations were set up at each study plot from subsequently checked every four days. Nests were considered early May to 15 July. Track stations were 1 m x 1 m squares of depredated if the nest was destroyed, indicating a predation event fine sand with a round white disc at the center to attract the had occurred. Nests were also considered depredated if there was attention of animals passing by. Tracks left in the sand when no indication of fledglings in the area and the nest was found animals investigated this novel object were photographed, empty more than one day before the predicted fledge date. Nests identified to species, and cleared every four days. A species was were considered successful if the nest was found empty within one considered present at a plot if an incidental observation was day of the predicted fledge date and parental behavior indicated recorded or if track station prints could be unambiguously fledglings were present in the area. identified to species. Nest predator detection Avian predators were surveyed with point counts. One point count Bushnell Trophy Cam trail cameras (model #119466C) were was conducted each day by each observer to prevent double deployed at 35 songbird nests to identify the species of nest counting birds at adjacent edge and interior study plots. To avoid predators in the region. Cameras were only deployed at nests with observer bias, observers rotated through study plots so that point complete clutches to minimize abandonment. Each camera was counts were conducted every eight days at each plot for a total of camouflaged with sage and grass and positioned at least 30 cm eight counts per plot. Each point count lasted 10 minutes and was from the nest. If the nest parents did not resume incubation or conducted within one hour of sunrise. Observers recorded all parental care within one hour following deployment, the camera birds detected by sight or sound within 100 m of the center of the was removed and installation was attempted on another visit. study plot (see Knight 2013 for details on point count methods). Camera batteries and 32 GB SD cards were changed every four The maximum number of individuals detected during a single Avian Conservation and Ecology 9(1): 8 http://www.ace-eco.org/vol9/iss1/art8/
point count was used as the abundance metric for each species were conducted to test for differences across the habitat types in (Nur et al. 1999). Birds that were not considered to be using the the abundance of known predator groups (mammal, bird, snake) habitat were not included in analysis. and predator species (Black-billed Magpies [Pica hudsonia], Western Meadowlarks, yellow-bellied racers [Coluber constrictor], Snake abundance was measured using standardized searches and and gopher snakes [Pituophis catenfir]). A fourth set of paired combined with incidental observations at each plot. Standardized Wilcoxon signed rank tests were conducted to test among the searches covered the entire plot during a 45 - 60 minute search habitat types for differences in local vegetative characteristics that every four days. Observers walked back and forth across the plot have previously been shown to be important for songbirds in in 10 m wide transects moving vegetation with a 1.5 m wooden sagebrush habitat (shrub height, shrub cover, grass height, exotic rod to detect any snakes present. The first plot to be searched at grass cover, native grass cover, native forb cover, standing litter each site was alternated between edge and interior after every height, litter cover, bare ground cover, biocrust cover). Finally, we search day to avoid any time of day bias. Incidental observations tested for differences in nest vegetation (grass height, distance to were recorded while observers were conducting other activities at the nearest shrub, visibility) between nests in edge habitat and each plot. Approximately equal time was spent at each plot to nests in interior habitat with Wilcoxon signed rank tests. avoid plot bias (35.8 ± 1.8 hours per plot), however snake abundance per species was divided by time spent at each plot to We used the logistic exposure method (Shaffer 2004) to estimate account for any variation. the predation-specific daily nest survival rate (PDSR) of songbirds in sagebrush habitat. The logistic exposure method Vegetation maximizes use of nest survival data by treating each nest check We measured the local vegetation at each plot along four 100 m interval as a discrete trial and uses a customized logistic link transects that spanned the width of the plot and were spaced 50 function in a generalized linear model (GLM) framework to allow m apart. We measured the percent linear cover of shrubs using for exposure periods to vary. We excluded any nests that failed the line intercept method (Kaiser 1983). We also recorded the because of abandonment, failure to hatch, or inclement weather maximum height of each shrub along each transect. We estimated because we were primarily interested in edge effects on predation percent cover of all forb and grass species, and percent cover of and because few nests failed because of causes other than ground cover type (bare ground, litter, i.e., dead vegetation, or predation. To ensure an adequate sample size, we pooled samples biocrust) with a standard 20 cm x 50 cm Daubenmire plot at every from all four study species, but considered the effect of species in 10 m along each transect (Daubenmire 1959). Maximum grass our model selection. We initially used a generalized linear mixed and standing litter height were also recorded at every 10 m along model (GLMM) framework to calculate PDSR because it allowed each transect. We averaged each vegetative characteristic to obtain us to account for any site and parental effects through the a value at each study plot. These vegetation survey methods follow inclusion of site and nest identity terms as random factors, but methods previously used in the study area to allow for direct ultimately chose to use a simpler GLM framework that allowed comparison in future studies (Paczek and Krannitz 2004, for calculation of Akaike information criterion (AIC) because the Harrison and Green 2010). Vegetation at each plot was measured GLMM and GLM frameworks provided similar results. The at the end of the breeding season to avoid disturbing any songbird length of the final nest check was constrained to the average day nests. a nest of that species would fledge (Dawson and Evans 1960, Baepler 1968, Rotenberry et al. 1999, Davis and Lanyon 2008). We also measured the vegetation around each nest after the nest had failed or successfully fledged young. We calculated percent We used an information theoretic approach to examine PDSR. visibility as the average visibility of the nest from one metre in To minimize the number of variables in any given model, we used four cardinal directions. We measured the maximum grass or forb a multistep variable inclusion approach to build a predictive height within 10 cm of the nest, and the distance to the nearest model of PDSR. Within each step, a global model was created shrub cover. with all terms of interest and competed against simpler models (Table 1). We assessed relative support using AIC values corrected Statistical analysis for small sample sizes (AICc). Steps one and two tested for any We tested for differences in nest abundance across habitat types species, camera, or temporal effects on PDSR that we might want before and after controlling for total songbird abundance. to control for in later model testing. The next three steps tested Songbird abundance was recorded during point counts conducted for effects of variables that could explain an edge effect on PDSR. for avian predators and was controlled for by dividing the number The third step of model selection evaluated whether predator of nests found in each plot by the summed abundance of the four abundance explained songbird nest fate. The fourth step of model study species in each plot. Total songbird abundance per species selection evaluated whether local (plot level) vegetation was measured as the maximum number of individuals detected characteristics explained nest fate. This step only considered in a single 10-minute point count. vegetation characteristics that varied between habitat type (see We compared both nest abundance metrics between interior above for details on paired Wilcoxon signed rank tests; Table 2) habitat and edge habitat (orchard edges and vineyard edges because we were interested in whether differences in nest fate combined) with a paired Wilcoxon signed rank test. To test for between habitat types were due to vegetation differences. The fifth differences between agricultural types, we compared orchard edge step examined whether nest fate was influenced by vegetation habitat to the paired orchard interior habitat, and vineyard edge characteristics at the nest site. In each step, we selected any habitat to the paired vineyard interior habitat with two additional variables that were present in at least two of the top three models paired Wilcoxon signed rank tests. A third set of similar analyses for further analysis (Table 3). Avian Conservation and Ecology 9(1): 8 http://www.ace-eco.org/vol9/iss1/art8/
Table 1. Terms considered to explain predation-specific daily nest survival rate (PDSR) and nestling condition of songbirds in sagebrush habitat. Terms were selected in a multistep hierarchical process using an information theoretic approach.
Term Class Description Levels Step 1. Species effects Species Factor Species of nest Brewer’s Sparrow, Lark Sparrow, Vesper Sparrow, Western Meadowlark Type Factor Type of nest substrate Shrub, Ground Camera Factor Presence or absence of nest camera on nest Present, Absent
Step 2. Temporal effects Day Integer Day of observation (May 1 = 1). Day2 Integer Squared term of Day Year Factor Year of observation 2011, 2012
Step 3a. Predator effects by group Mammal Integer Sum of mammalian predator species Bird Integer Sum of max point count abundance of avian predators Snake Numeric Snakes detected per survey minute Mice Numeric Proportion of track tubes with mouse prints
Step 3b. Predator effects by species BBMA Integer Maximum point count abundance of Black-billed Magpies WEME Integer Maximum point count abundance of Western Meadowlarks Racer Numeric Yellow-bellied racers seen per minute Gopher Numeric Gopher snakes seen per minute
Step 4. Plot vegetation effects Exotic Grass% Numeric Percent cover of exotic grasses Native Grass% Numeric Percent cover of native grasses Shrub% Numeric Percent cover of shrubs ShrubHt Numeric Average height of shrubs
Step 5. Nest vegetation effects GrassHt Numeric Maximum grass height at nest ShrubD Numeric Distance to the nearest shrub Visibility Numeric Percent visibility from 1 m averaged over four cardinal directions Brewer’s Sparrows (Spizella breweri), Lark Sparrows (Chondestes grammacus), Vesper Sparrows (Pooecetes gramineus), Western Meadowlarks (Sturnella neglecta), Black-billed Magpies (Pica hudsonia), Yellow-bellied racers (Coluber constrictor), gopher snakes (Pituophis catenfir)
Next we tested for an edge effect on PDSR by comparing four the multistep inclusion process explained any of this edge effect models each representing a different edge-effect hypothesis (Black-billed Magpie abundance, study plot shrub cover, grass including a null model (Table 4). These edge-effect hypotheses height at nest). We added the complete set of terms to the orchard included a nest stage term to control for higher PDSR during the and null hypothesis models. We also added each term separately incubation stage. Estimates of PDSR in orchard edge habitat and to the two hypothesis models to examine the effects of each in vineyard edge/interior habitat were calculated by bootstrapping variable separately. Competing the simpler models with the full the data 10000 times and fitting it to the model representing the models was also important for choosing the model with the most orchard edge hypothesis. We created each bootstrap replicate by support because AIC penalizes models with many parameters, resampling from our sample of nests rather than resampling from and although some of the final terms included met our criteria our sample of nest checks. for inclusion, they had little ∆ICC support. An overall estimate of PDSR for the population was calculated by bootstrapping the Upon finding strong support for an orchard edge effect on PDSR entire sample of nests and fitting it to the most supported final over all other models, we tested whether any variables selected in model. Avian Conservation and Ecology 9(1): 8 http://www.ace-eco.org/vol9/iss1/art8/
Table 2. Local vegetation characteristics in edge and interior sagebrush habitat. Edge habitat was located adjacent to vineyard (n = 8) or orchard (n = 10). All sites indicates orchard and vineyard sites pooled. Differences between edge and interior sites were tested with paired Wilcoxon rank sign tests.
All sites Orchard sites Vineyard sites Vegetation Edge Interior U P Edge Interior U P Edge Interior U P char. habitat habitat habitat habitat habitat habitat mean mean mean mean mean mean ± se ± se ± se ± se ± se ± se Shrub height 100.56 77.35 163 <0.01 110.04 82.26 52 0.01 105.06 97.00 35 0.02 (cm) ± 1.95 ± 6.77 ± 9.00 ± 9.79 ± 8.85 ± 9.29 Shrub cover 28.80 23.53 139 0.02 34.30 28.81 46 0.06 21.94 16.92 28 0.19 (%) ± 3.07 ± 2.85 ± 4.34 ± 3.90 ± 3.04 ± 2.93 Grass height 38.48 36.06 113 0.25 37.24 37.09 33 0.62 40.02 34.77 26 0.31 (cm) ± 1.61 ± 1.52 ± 2.57 ± 2.16 ± 1.72 ± 2.18 Exotic grass 19.83 11.38 143 0.01 20.79 9.84 49 0.03 18.63 13.30 27 0.25 cover (%) ± 3.02 ± 2.35 ± 4.33 ± 3.49 ± 4.40 ± 3.12 Native grass 14.94 18.96 42 0.06 14.47 21.22 3 0.01 15.52 16.12 17 0.95 cover (%) ± 1.70 ± 2.01 ± 2.21 ± 3.25 ± 2.79 ± 1.71 Native forb 15.74 18.07 58 0.25 15.00 15.60 21 0.56 16.65 21.16 10 0.31 cover (%) ± 1.70 ± 1.83 ± 2.69 ± 2.11 ± 1.99 ± 2.95 Standing litter 9.51 10.40 63 0.35 9.51 12.29 15 0.23 9.52 8.04 18 1.06 height (cm) ± 1.88 ± 1.88 ± 2.69 ± 2.90 ± 2.79 ± 2.07 Litter cover (%) 64.45 61.16 114 0.23 63.45 61.75 33 0.62 65.71 60.42 28 0.20 ± 3.29 ± 3.81 ± 5.48 ± 5.32 ± 3.23 ± 5.81 Bare ground 13.13 16.06 66 0.42 11.84 16.69 13 0.16 14.75 15.28 22 0.64 cover (%) ± 1.62 ± 2.47 ± 2.01 ± 3.76 ± 2.68 ± 3.22 Biocrust cover 14.70 17.70 74 0.64 15.39 16.14 30 0.85 13.85 19.65 11 0.38 (%) ± 1.95 ± 2.73 ± 2.77 ± 2.59 ± 2.87 ± 5.38
All statistical analyses were conducted in R version 2.15 using the types, vineyard edge habitat had fewer per capita nests than the packages AICcmodavg for calculation of AICc, and respective paired interior habitat (U7 = 4, P = 0.05). Orchard edge exactRankTests for paired Wilcoxon signed rank tests (R Core and paired interior habitat had similar per capita abundances of
Team 2012, Hothorn and Hornik 2013, Mazerolle 2013). Code nests (U9 = 9, P = 0.13). for the specialized link function used in the logistic exposure method was obtained from the U.S. Geological Survey Northern Nest predator detection Prairie Research Center (M. Herzog, http://www.npwrc.usgs.gov/ Ten of the 35 nests monitored with cameras were depredated. We resource/birds/nestsurv/download/CreateLogisticExposureFamily. identified predators in eight predation events at seven of these R). nests (one nest was partially depredated twice by two different snake species; Table 6). We observed two additional predation RESULTS events at nests where cameras were not deployed. Two species of snake, two species of birds, and two species of mammal were Nest abundance identified as nest predators. Snakes, particularly yellow-bellied Seventy five nests were found and monitored (Table 5). Overall, racers, were the most common predator of songbird nests in fewer nests were found per plot in edge habitat than in interior sagebrush shrubsteppe. habitat (U18 = 20, P = 0.01; Fig. 2A). When agricultural types were considered separately, there were far fewer nests found in Nest predator abundance The abundance of large mammal predators, bird predators, and orchard edge habitat than the paired interior habitat (U9 = 6, P = 0.07). Vineyard edge habitat did not have significantly fewer mice was similar in edge and interior habitat (Table 7). There were more snakes in edge than interior habitat. When we analyzed nests than paired interior habitat (U7 = 5.5, P = 0.17; but see Fig. 2A). This lack of edge effect was in part due to the presence of a individual predator species, we found yellow-bellied racers were vineyard edge outlier where there were multiple Lark Sparrows, more abundant in orchard edge habitat, and vineyard edge habitat whose nests are easier to find. After removal of this outlier, there than the corresponding interior habitats. In contrast, the were significantly fewer nests in vineyard edge habitat than in abundance of gopher snakes was similar between all habitat types. The abundance of Black-billed magpies and Western vineyard interior habitat (U6 = 0, P = 0.03). Meadowlarks in edge and interior habitat did not differ, although Overall, there were fewer per capita nests in edge habitat than there was a suggestion that Black-billed magpies were less interior habitat (U17 = 28, P = 0.02; Fig. 2B). Among agricultural abundant in orchard edge habitat than orchard interior plots. Avian Conservation and Ecology 9(1): 8 http://www.ace-eco.org/vol9/iss1/art8/
Fig. 2. A. Difference in the number of songbird nests found in Table 3. AICC ranking of logistic-exposure models for predation- edge and interior sagebrush habitat. Difference in the number specific daily nest survival rate of songbirds nesting in sagebrush of nests was calculated as the number of nests in each edge plot habitat. A multistep variable inclusions approach was used to minus the number of nests in the paired interior plot. B. minimize the number of variables in any given model. The top Difference in the number of per capita songbird nests found in three models for each step are shown. See Table 1 for terms. edge and interior sagebrush habitat. Difference in per capita nests was calculated as the mean nests per songbird in each Model N K AICC ∆AICC wi edge plot minus the mean nests per songbird in the paired Step 1. Species effects (8 models) interior plot. Edge habitat was located adjacent to vineyard (n Fate = Null 209 1 166.82 0.00 0.42 = 8) or orchard (n = 10). All sites includes orchard and Fate = Cam 208 2 168.12 1.30 0.22 vineyard sites pooled. Box ends represent the 25th and 75th Fate = Type 208 2 168.54 1.72 0.18 percentiles, whiskers represent 1.5 interquartile range, and dots represent outliers in all boxplots. Step 2. Temporal effects (12 models) Fate = Stage 208 2 165.92 0.00 0.19 Fate = Null 209 1 166.68 0.76 0.13 Fate = Stage + Year 207 3 166.82 0.90 0.12
Step 3a. Predator effects by group (16 models) Fate = Null 209 1 166.82 0.00 0.19 Fate = Mammal 208 2 167.22 0.40 0.16 Fate = Mice 208 2 168.38 1.56 0.09
Step 3b. Predator effects by species (16 models) Fate = BBMA 208 2 166.00 0.00 0.17 Fate = BBMA + Racer 207 3 166.24 0.25 0.15 Fate = Null 209 1 166.82 0.82 0.11
Step 4. Plot vegetation effects (16 models) Fate = Shrub% 208 2 161.80 0.00 0.32 Fate = Shrub% + Native 207 3 163.62 1.83 0.13 Grass% Fate = Shrub% + Exotic 207 3 163.83 2.03 0.12 Grass%
Step 5. Nest vegetation effects (20 models) Fate = GrassHt 208 2 163.70 0.00 0.40 Fate = GrassHt + Visibility 207 3 165.04 1.34 0.20 Fate = GrassHt + ShrubD 207 3 165.75 2.05 0.14
Nest grass height and visibility was similar in edge and interior habitat (all P > 0.05). Nests in edge habitat were marginally further
from shrubs than nests in interior habitat (U63 = 513.5, P = 0.08). Nest predation Of the 75 nests found, we determined the fate of 72. Young were fledged at 37 nests and 27 nests were depredated (Table 5). The remaining nine nests failed because of abandonment following camera placement (n = 3), abandonment following apparent Vegetation partial predation (n = 2), abandonment following brood Grass height, native forb cover, standing litter height, litter cover, parasitism (n = 1), failure to hatch (n = 1), or inclement weather bare ground cover, and biocrust cover were similar between all (n = 2). Predation was therefore responsible for 75% of nest habitat types (all P > 0.05; Table 2). Orchard edge habitat had failures. higher shrub cover, higher exotic grass cover, and lower native Songbird species, type of nest (ground or shrub), and the presence grass cover than interior habitat, although the difference in shrub of a nest camera did not impact PDSR (Table 4). PDSR was lower cover was marginal. Local vegetation in vineyard edge habitat during the incubation stage, and the nest stage model had an was similar to local vegetation in vineyard interior habitat, with evidence ratio of 1.58 over the null model. Predator abundance the exception of shrub height, which was greater in both types of at the group level did not influence PDSR, but when examined at agricultural edge habitat than interior habitat. Avian Conservation and Ecology 9(1): 8 http://www.ace-eco.org/vol9/iss1/art8/
Fig. 3. Logistic exposure estimates of predation-specific daily nest survival rates of songbirds nesting in sagebrush shrubsteppe habitat. Solid lines indicate the logistic exposure estimate and dotted lines are the 95% CI. Nests were monitored in three habitat types: orchard edge habitat, vineyard edge habitat, and interior habitat, however we pooled vineyard edge and interior habitat because general linear modelling in an information theoretic framework indicated there was no difference in nest fate between vineyard and interior habitat. Estimates were calculated using three different models each including a habitat type term, a nest stage term (incubation and nestling estimates averaged here) and one of: Black-billed Magpie (Pica hudsonia) abundance (A), percent shrub cover (B), and grass height at nest (C).
the species level, increased Black-billed Magpie abundance was representing a difference in PDSR in orchard edge habitat, with associated with decreased PDSR (Figure 3), although the top an evidence ratio of 4.6 over the null model. The other models representing a vineyard edge effect and general edge effect Table 4. Hypotheses developed to test for an edge effect on hypothesis had less support than the null model (Table 8). Despite songbird nest predation in sagebrush shrubsteppe habitat at two the small number of nests in the orchard edge habitat, different types of agricultural edges: orchard and vineyard. bootstrapped estimates of PDSR calculated by resampling from our sample of nests within each habitat had confidence intervals Term Hypothesis Levels with minimal overlap (orchard edge PDSR = CI etc. The set of Edge Nest fate differs between edge and interior Edge, bootstrapped data fitted to the orchard edge hypothesis model habitat Interior estimated PDSR was 0.860 (0.685 - 0.946 95% CI) in orchard edge Orchard Nest fate does not differ between vineyard Orchard habitat and 0.966 (0.941 - 0.981 95% CI) in interior and vineyard edge and interior habitat, but does differ in Edge, habitat (Fig. 3A). orchard edge habitat Vineyard/ Interior After inclusion of the mechanism variables to the null and orchard Vineyard Nest fate does not differ between orchard Vineyard models, the orchard edge term was included in four of the top edge and interior habitat, but does differ in Edge, five models, and had a summed wi of 0.74. Vegetation differences vineyard edge habitat Orchard/ at the study plot or nest site therefore do not completely explain Interior the orchard edge effect on PDSR. Nevertheless our analysis Null Nest fate does not differ between orchard None provides some evidence that shrub cover within the study plot edge, vineyard edge, and interior habitat explains some of the orchard effect because the model including orchard and shrub cover had only slightly more support than the model including Black-billed Magpie abundance only had an model including shrub cover alone (evidence ratio = 1.14, ∆ICC evidence ratio of 1.54 over the null model. Among vegetation = 0.32; Table 8). Increasing shrub cover was associated with a characteristics at the plot level, the percent cover of shrubs was decrease in PDSR (Fig. 3B). Similarly, we found some evidence present in the top seven models, and was a strong negative that grass height at the nest site explains some of the orchard predictor of PDSR with an evidence ratio of 10.66 over the null effect because the model including orchard and grass height at model. At the nest site, grass height was present in the top four the nest site had only slightly more support than the model with models, and was a strong positive predictor of PDSR with an grass height alone (evidence ratio = 1.50, ∆ICC = 0.93). Increasing evidence ratio of 5.00. nest site grass height was associated with an increase in PDSR (Fig. 3C). A post-hoc Wilcoxon rank sum test showed that nest We found evidence for edge effects on PDSR in one agricultural grass height is marginally lower at nests in orchard edge habitat edge habitat type. There was strong support for the model than at nests in vineyard or interior habitat (U63 = 278.5, P = Avian Conservation and Ecology 9(1): 8 http://www.ace-eco.org/vol9/iss1/art8/
Table 5. Numbers of songbirds detected, songbird nests found (including fate), and nest predator identifications in three types of sagebrush habitat. Songbird species studied were Brewer’s Sparrows, Spizella breweri (BRSP), Lark Sparrows, Chondestes grammacus (LASP), Vesper Sparrows, Pooecetes gramineus (VESP), and Western Meadowlarks, Sturnella neglecta (WEME).
Variable measured Orchard edge habitat Vineyard edge habitat Interior habitat (n = 10) (n = 8) (n = 18) Mean ± se Total Mean ± se Total Mean ± se Total Maximum number of birds detected on a single point count BRSP 0.60 ± 0.31 6 0.12 ± 0.12 1 1.22 ± 0.46 22 LASP 0.80 ± 0.20 8 1.34 ± 0.18 11 0.61 ± 0.20 11 VESP 1.20 ± 0.25 12 2.62 ± 0.42 21 3.17 ± 0.32 57 WEME 1.60 ± 0.45 16 2.40 ± 0.65 19 2.17 ± 0.53 39 Total 4.20 ± 0.73 42 6.50 ± 0.87 52 7.17 ± 0.58 129
Nests found BRSP 0.10 ± 0.10 1 0.00 ± 0.00 0 0.78 ± 0.50 14 LASP 0.50 ± 0.22 5 0.50 ± 0.38 4 0.44 ± 0.29 8 VESP 0.20 ± 0.20 2 0.62 ± 0.26 5 1.56 ± 0.32 28 WEME 0.10 ± 0.10 1 0.50 ± 0.27 4 0.17 ± 0.38 3 Total 0.90 ± 0.28 9 1.62 ± 0.60 13 2.94 ± 0.70 53
Fate unknown 1 0 2 Abandoned 1 0 5 Failed because of weather 0 0 2 Predated 5 4 18 Fledged 2 9 26
Nest predator identification sample sizes Nest cameras deployed 4 7 24 Predator identifications 2 3 5
0.09). There was little relationship between study plot shrub cover edge habitat than interior sagebrush habitat. We also found and nest grass height in post-hoc Pearson’s test for correlation (r increased predation rates in sagebrush habitat adjacent to = -0.22, n = 61, p = 0.02), suggesting these two vegetation orchards partially because of differences in vegetation. characteristics each explain different portions of the observed Our study finds that songbirds in sagebrush edge habitat suffer orchard edge effect on nest predation. reproductive impacts that are not detected with abundance Overall, PDSR was best described by the orchard edge hypothesis, estimates, and supports previous reports of sagebrush edge nest stage, and the abundance of Black-billed Magpies, and the avoidance (Ingelfinger and Anderson 2004). We found fewer total orchard edge hypothesis term (Table 8). This top model estimated nest attempts in both types of agricultural edge habitat than their overall PDSR as 0.958 (0.915 - 0.980 95% CI). respective interior habitat. When we compared per capita nesting attempts, however, we only detected a difference between paired DISCUSSION vineyard edge and interior habitats, suggesting that the edge Fragmentation of sagebrush landscapes has been suggested to effects on nest abundance differ between the two edge types we contribute to observed declines of songbirds in sagebrush habitat studied. The edge effect on nest abundance in orchard edge habitat (Knick and Rotenberry 2002, Knick et al. 2003, Vander Haegen appears to be due to edge avoidance: individuals, particularly 2007). Songbirds in fragmented sagebrush landscapes experience Vesper and Brewer’s Sparrows, do not occupy orchard edge higher rates of nest predation and modeling suggests this habitat as frequently as interior habitat, and so the relative contributes to negative population growth in fragmented abundance of nests found in orchard edge habitat is also lower. landscapes (Vander Haegen et al. 2002, Vander Haegen 2007). Grassland songbirds have varied responses to the presence of Several authors have suggested these types of fragmentation edges (Helzer and Jelinski 1999, Ribic et al. 2009), but birds in effects detected at large scales are caused by the cumulative impact sagebrush shrubsteppe generally show sensitivity to habitat of edge effects across the landscape (Bender et al. 1998, Ries et fragmentation (Knick and Rotenberry 1995, Ingelfinger and al. 2004, Fletcher et al. 2007). We showed here that there are Anderson 2004, Vander Haegen 2007). Songbirds likely avoid multiple reproductive edge effects on songbirds breeding in edges with stronger vegetative contrast, and songbirds in sagebrush habitat. There was a reduction in the number of overall sagebrush habitat in the study region have previously been shown nesting attempts and nesting attempts per songbird in agricultural to avoid areas with high tree density (Krannitz 2007). We suggest Avian Conservation and Ecology 9(1): 8 http://www.ace-eco.org/vol9/iss1/art8/
Table 6. Details of confirmed predation events at songbird nests in sagebrush habitat. Predation events were considered confirmed if recorded by nest camera or visually observed.
Predator species Observation type Date Time Songbird species Study habitat type Domestic cow Camera 2012-07-29 19:10 Vesper Sparrow Interior (Bos primigenius) (Pooecetes gramineus) Unidentified mammal Camera 2011-06-18 02:25 Lark Sparrow Orchard edge (Chondestes grammacus) Mammal total: 2
Black-billed Magpie Camera 2012-06-12 09:33 Vesper Sparrow Interior (Pica hudsonia) Western Meadowlark (Sturnella Camera 2012-06-09 10:13 Lark Sparrow Vineyard edge neglecta) Western Meadowlark Visual 2012-05-29 07:43 Vesper Sparrow Interior Bird total: 3
Yellow-bellied racer Camera 2011-07-03 10:05 Vesper Sparrow Interior (Coluber constrictor) Yellow-bellied racer Camera 2011-06-19 20:01 Vesper Sparrow Interior Yellow-bellied racer Camera 2011-06-29 11:23 Vesper Sparrow Vineyard edge Yellow-bellied racer Visual 2011-06-04 08:23 Brewer’s Sparrow Orchard edge (Spizella breweri) Gopher snake Camera 2011-06-27 16:48 Vesper Sparrow Vineyard edge (Pituophis catenfir) Snake total: 5
songbirds in sagebrush habitat avoid treed orchard edges, but not nest predation rates suggests snakes use vineyard edge habitat for more shrub-like vineyard edges. Alternatively, the edge effect on activities other than foraging. Two similar studies also concluded nest abundance in vineyard edge habitat could be due to reduced that snakes were not foraging in edge habitat where they were observer ability to find nests or fewer per capita nesting attempts. most abundant because they did not cause higher predation rates Although we did choose paired plots with similar vegetation, there of songbird nests (Sperry et al. 2009, Weatherhead et al. 2010). was slightly higher native bunchgrass cover in vineyard edge There is anecdotal evidence that snakes in this region are traveling habitat, but we do not believe this was enough to reduce the through vineyard edge habitat to the adjacent vineyards because number of nests found. Instead, we suggest a vineyard edge effect they are attracted to high rodent abundance and irrigated areas on per capita nest attempts is due to reduced pairing success. We (C. Bishop, personal communication), while snakes are likely recorded multiple male Vesper Sparrows, the most common bird residing and foraging in orchard edge habitat. Increased in our study sites, whose territories included vineyard habitat and abundance of another known predator, the Black-billed Magpie, who did not appear to attract mates. Ovenbirds (Seiurus did decrease daily nest survival, but did not explain the edge effect aurocapilla) that reside closer to forest edges also have reduced on nest predation because Black-billed Magpies were more pairing success perhaps because of an abundance of first-time abundant in interior than orchard edge habitat. breeders in edge habitat (Van Horn et al. 1995, Bayne and Hobson Another possible explanation for an edge effect on nest predation 2001). In combination, the reduced total nest abundance in is differences in vegetation that make it easier for predators to orchard edge habitat and reduced per capita nest abundance in locate nests in edge habitat (Smith 2004). We found that lower vineyard edge habitat detected in this study suggest sagebrush grass height at the nest and increased shrub cover in the study habitat adjacent to agriculture is of poor quality for songbirds. plot both explained portions of the edge effect on nest predation Edge effects on nest predation are most often attributed to higher observed at orchard edges. Grassland songbirds often pick nest nest predator abundance in edge habitat, such as the higher sites with taller grass, likely because higher vegetation conceals predation rate we detected in orchard edge, but not vineyard edge nests better (Davis 2005). This concealment theory is supported habitat (Chalfoun et al. 2002, Smith 2004). Contrary to by many studies finding a relationship between grass height at the expectation, nest predator abundance did not explain the edge nest and nest survival (Davis 2005, Klug et al. 2010, Stauffer et effect on nest predation in orchard edge habitat here. Snakes, the al. 2011). The predators detected by nest cameras here, dominant nest predators, were found most frequently in vineyard particularly snakes, Black-billed Magpies, and Western edge habitat, whereas orchard edge habitat had the highest nest Meadowlarks, are visual predators that could benefit increased predation rate. This discrepancy between snake abundance and foraging efficiency from reduced nest concealment (Weatherhead Avian Conservation and Ecology 9(1): 8 http://www.ace-eco.org/vol9/iss1/art8/
Table 7. The relative abundance of potential predators of songbird nests in edge and interior sagebrush habitat. Predators were analyzed by major taxonomical group and as individual species. Edge habitat was located adjacent to vineyard (n = 8) or orchard (n = 10). All sites includes orchard and vineyard sites pooled. Differences between edge and interior sites were tested with paired Wilcoxon rank sign tests.
All sites Orchard sites Vineyard sites Predator Edge Interior U P Edge Interior U P Edge Interior U P habitat habitat habitat habitat habitat habitat mean mean mean ± mean ± mean ± mean ± ± se ± se se se se se Predator group Birds 3.72 3.50 75.5 0.72 3.20 2.70 29 0.51 4.38 4.50 13 0.94 ± 0.54 ± 0.73 ± 0.51 ± 0.97 ± 1.03 ± 1.07 Large mammal 2.11 1.83 60.5 0.31 2.20 1.70 22 0.19 2.00 2.00 10.5 1.19 species ± 0.24 ± 0.25 ± 0.36 ± 0.26 ± 0.33 ± 0.46 Mice 0.31 0.27 89 0.58 0.31 0.31 26 0.92 0.31 0.23 20 0.38 ± 0.07 ± 0.07 ± 0.07 ± 0.10 ± 0.13 ± 0.09 Snakes 0.10 0.05 141 <0.001 0.08 0.05 38 0.07 0.13 0.05 36 0.01 ± 0.02 ± 0.01 ± 0.02 ± 0.01 ± 0.03 ± 0.02
Predator species Black-billed 0.39 0.89 11.5 0.12 0.20 1.10 0 0.06 0.62 0.62 7.5 1.25 Magpie (Pica ± 0.14 ± 0.24 ± 0.13 ± 0.38 ± 0.26 ± 0.26 hudsonia) Gopher snake 0.02 0.02 41 0.79 0.01 0.02 4 0.22 0.03 0.02 18 0.58 (Pituophis ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 ± 0.01 catenfir) Western 1.94 2.17 32.5 0.63 1.60 1.50 8.5 1.00 2.38 3.00 9.5 0.55 Meadowlark ± 0.38 ± 0.53 ± 0.45 ± 0.62 ± 0.65 ± 0.87 (Sturnella neglecta) Yellow-bellied 0.09 0.03 153 <0.001 0.07 0.03 45 0.003 0.10 0.03 36 0.01 racer (Coluber ± 0.01 ± 0.01 ± 0.02 ± 0.01 ± 0.02 ± 0.02 constrictor)
and Blouin Demers 2004, Davis and Lanyon 2008, Birkhead sizes, particularly in orchard edge habitat. Although the sample 2010). A negative relationship between shrub cover and nest sizes are small, they were adequate to detect other biologically survival has also been detected for grassland songbirds, and is relevant impacts, such as variation in nest predation rates with thought to occur because shrubs provide cover or habitat for nest nest stage (e.g., Renfrew and Ribic 2003). Our sample sizes were predators (With 1994). In particular, nest predation risk by snakes also large enough to detect vegetation and predator effects on nest is higher in areas with more shrubs because the snakes likely use predation rate. Despite accounting for all of these potential shrubs for thermoregulation (Klug et al. 2010). We suggest factors, our information theoretic approach still finds support for orchard edge habitat in the Okanagan is characterized by high an edge effect on nest predation. We have also analyzed the data predation rates because of an interaction between snake behavior in multiple ways, and remain confident the nest survival data are and high shrub cover, but we were unable to test for this interaction indicative of a real trend in orchard edge habitat. because of our small orchard edge sample size. We are unsure why Similarity between the daily nest survival rate found here and shrub cover is particularly high in orchard edge habitat, but daily nest survival rates of songbirds in sagebrush habitat suggest it could result from deeper soils near agricultural areas calculated in other fragmented landscapes with negative (Vander Haegen et al. 2000, Iverson et al. 2008). Regardless, population growth (Vander Haegen 2007) suggests that further investigation is required to elucidate whether the reproduction in the Okanagan area is insufficient to maintain vegetation effects on nest predation detected here can be songbird populations in sagebrush habitat. Negative population generalized to all orchard edge habitat in the study region. The trends have been detected in the study region for all four songbird only previous edge effect on nest predation study in sagebrush species studied here, and edge effects on reproduction may be shrubsteppe habitat did not find higher nest predation adjacent contributing to these declines (Sauer et al. 2014). We recommend to agriculture (Vander Haegen et al. 2002), but their study several management steps that can be taken to mitigate edge addressed a different crop type. effects on songbird reproduction in sagebrush habitat. First, We remain confident in our conclusion that nest predation rates conservation areas for songbirds in Okanagan sagebrush habitat are higher in orchard edge habitat despite relatively small sample should be placed away from agricultural areas to avoid negative Avian Conservation and Ecology 9(1): 8 http://www.ace-eco.org/vol9/iss1/art8/
Table 8. AICC ranking of logistic-exposure models for predation-specific daily nest survival rate of songbirds nesting in sagebrush habitat. Four specific edge-effect hypotheses were initially tested (see Table 3), and then potential explanatory mechanism variables were added for final model selection. All models tested are shown. See Table 1 for terms.
Model N K AICC ∆AICC wi Edge effect hypotheses Fate = Orchard + Stage 207 3 162.61 0.00 0.72 Fate = Stage 208 2 165.92 3.31 0.14 Fate = Vineyard + Stage 207 3 167.25 4.64 0.07 Fate = Edge + Stage 207 3 167.31 4.70 0.07
Final model selection Fate = Orchard + BBMA + Stage 206 4 159.05 0.00 0.36 Fate = Orchard + BBMA + GrassHt + Shrub% + Stage 204 6 159.74 0.70 0.25 Fate = BBMA + GrassHt + Shrub% + Stage 205 5 160.91 1.86 0.14 Fate = Orchard + GrassHt + Stage 206 4 162.24 3.19 0.07 Fate = Orchard + Shrub% + Stage 206 4 162.60 3.56 0.06 Fate = Shrub% + Stage 207 3 162.60 3.56 0.06 Fate = GrassHt + Stage 207 3 164.00 4.96 0.03 Fate = BBMA + Stage 207 3 165.12 6.07 0.02 reproductive impacts of habitat edges on songbirds in sagebrush LITERATURE CITED habitat. To prevent edge avoidance and potentially higher nest Andrén, H. 1995. Effects of landscape composition on predation predation rates in orchard edge habitat, managers should avoid rates at habitat edges. Pages 225-255 in L. Hansson, L. Fahrig, the creation of new orchard-sagebrush habitat edges or buffer and G. Merriam, editors. Mosaic landscapes and ecological orchard edges and encourage the presence of vineyards at the processes. Chapman & Hall, London, UK. periphery of the agricultural matrix. In light of the different edge effects adjacent to different crop types detected here, we Andrén, H., P. Angelstam, E. Lindström, and P. Widén. 1985. discourage future agricultural edge effect studies from grouping Differences in predation pressure in relation to habitat agricultural edge types together. If crop types are not separated, fragmentation: an experiment. Oikos 45:273-277. http://dx.doi. edge effects may not be detected, or the results may be org/10.2307/3565714 unnecessarily generalized and ineffective management taken. Baepler, D. H. 1968. Lark sparrow. Pages 886-902 in O. L. Austin, editor. Life histories of North American cardinals, grosbeaks, Responses to this article can be read online at: buntings, towhees, finches, sparrows, and allies. Bulletin of the United States National Museum, Washington, D.C., USA. http://www.ace-eco.org/issues/responses.php/662 Batáry, P., and A. Báldi. 2004. Evidence of an edge effect on avian nest success. Conservation Biology 18:389-400. http://dx.doi. org/10.1111/j.1523-1739.2004.00184.x Acknowledgments: Bayne, E. M., and K. A. Hobson. 2001. Effects of habitat Thank you to three anonymous reviewers whose detailed comments fragmentation on pairing success of ovenbirds: importance of greatly improved the first version of this manuscript, and to Dr. male age and floater behavior. Auk 118:380-388. http://dx.doi. Elizabeth Elle who provided comments to the thesis version of the org/10.1642/0004-8038(2001)118[0380:EOHFOP]2.0.CO;2 manuscript. This research could not have been conducted without the dedicated assistance of field technicians Gregory Rickbeil, Sonia Bender, D. J., T. A. Contreras, and L. Fahrig. 1998. Habitat loss Nicholl, Hannah Gehrels, Megan McAndrew, and Sebastián Pardo. and population decline: a meta-analysis of the patch size effect. Funding and equipment for this project was provided by Environment Ecology 79:517-533. http://dx.doi.org/10.1890/0012-9658(1998) Canada, the Centre for Wildlife Ecology, the Natural Sciences and 079[0517:HLAPDA]2.0.CO;2 Engineering Research Council, the Nature Trust, Bushnell, and Benson, T. J., J. D. Brown, and J. C. Bednarz. 2010. Identifying Simon Fraser University. Thank you to the staff of The Land predators clarifies predictors of nest success in a temperate Conservancy, the Environmental Farm Plan, the South Okanagan- passerine. Journal of Animal Ecology 79:225-234. http://dx.doi. Similkameen Conservation Program, and the Washington org/10.1111/j.1365-2656.2009.01604.x Department of Fish and Wildlife for invaluable landowner contact assistance. We sincerely thank the many private landowners who Birkhead, T. 2010. The magpies: the ecology and behaviour of graciously granted us access to their land to conduct this research. Black-billed and Yellow-billed Magpies. A & C Black, London, UK. Avian Conservation and Ecology 9(1): 8 http://www.ace-eco.org/vol9/iss1/art8/
Chalfoun, A. D., F. R. Thompson, and M. J. Ratnaswamy. 2002. Glennon, M. J., W. F. Porter, and C. L. Demers. 2002. An Nest predators and fragmentation: a review and meta-analysis. alternative field technique for estimating diversity of small- Conservation Biology 16:306-318. http://dx.doi.org/10.1046/ mammal populations. Journal of Mammalogy 83:734-742. http:// j.1523-1739.2002.00308.x dx.doi.org/10.1644/1545-1542(2002)083<0734:AAFTFE>2.0.CO;2 Cox, W. A., F. R. Thompson, and J. Faaborg. 2012. Landscape Harrison, M. L., and D. J. Green. 2010. Vegetation influences forest cover and edge effects on songbird nest predation vary by patch occupancy but not settlement and dispersal decisions in a nest predator. Landscape Ecology 27:659-669. http://dx.doi. declining migratory songbird. Canadian Journal of Zoology org/10.1007/s10980-012-9711-x 88:148-160. http://dx.doi.org/10.1139/Z09-125 Daubenmire, R. 1959. A canopy-coverage method of vegetational Hartley, M. J., and M. L. Hunter. 1998. A meta-analysis of forest analysis. Northwest Science 33:43-64. cover, edge effects, and artificial nest predation rates. Conservation Biology 12:465-469. Davis, S. K. 2005. Nest-site selection patterns and the influence of vegetation on nest survival of mixed-grass prairie passerines. Helzer, C. J., and D. E. Jelinski. 1999. The relative importance of Condor 107:605-616. http://dx.doi.org/10.1650/0010-5422(2005) patch area and perimeter-area ratio to grassland breeding birds. 107[0605:NSPATI]2.0.CO;2 Ecological Applications 9:1448-1458. Davis, S. K., R. M. Brigham, T. L. Shaffer, and P. C. James. 2006. Hothorn, T., and K. Hornik. 2013. exactRankTests: Exact Mixed-grass prairie passerines exhibit weak and variable distributions for rank and permutation tests. The R Project for responses to patch size. Auk 123:807-821. http://dx.doi. Statistical Computing, Vienna, Austria. org/10.1642/0004-8038(2006)123[807:MPPEWA]2.0.CO;2 Ingelfinger, F., and S. Anderson. 2004. Passerine response to roads Davis, S. K., and W. E. Lanyon. 2008. Western Meadowlark associated with natural gas extraction in a sagebrush steppe (Sturnella neglecta). In A. Poole, editor. Birds of North America, habitat. Western North American Naturalist 64:385-395. Number 104. Cornell Lab of Ornithology, Ithaca, NY, USA. Iverson, K., D. Curran, T. Fleming, and A. Haney. 2008. Sensitive Dawson, W. R., and F. C. Evans. 1960. Relation of growth and ecosystems inventory Okanagan Valley: Vernon to Osoyoos 2000 - development to temperature regulation in nestling Vesper 2007. Pacific and Yukon Region Canadian Wildlife Service, Delta, Sparrows. Condor 62:329-340. http://dx.doi.org/10.2307/1365163 British Columbia, Canada. Dobler, F. C., J. Eby, C. Perry, S. Richardson, and W. M. Vander Kaiser, L. 1983. Unbiased estimation in line-intercept sampling. Haegen. 1996. Status of Washington’s shrub-steppe ecosystem: Biometrics 39:965-976. http://dx.doi.org/10.2307/2531331 extent, ownership, and wildlife/vegetation relationships. Washington Klug, P. E., S. L. Jackrel, and K. A. With. 2010. Linking snake Department of Fish and Wildlife, Olympia, Washington, USA. habitat use to nest predation risk in grassland birds: the dangers Donovan, T. M., P. W. Jones, E. M. Annand, and F. R. Thompson of shrub cover. Oecologia 162:803-813. http://dx.doi.org/10.1007/ III. 1997. Variation in local-scale edge effects: mechanisms and s00442-009-1549-9 landscape context. Ecology 78:2064-2075. http://dx.doi. Knick, S. T., D. S. Dobkin, J. T. Rotenberry, M. A. Schroeder, W. org/10.1890/0012-9658(1997)078[2064:VILSEE]2.0.CO;2 M. Vander Haegen, and C. Van Riper III. 2003. Teetering on the Environmental Systems Research Institute (ESRI). 2011. ArcMap edge or too late? Conservation and research issues for avifauna 10.0. Environmental Systems Research Institute, Redlands, of sagebrush habitats. Condor 105:611-634. http://dx.doi. California, USA. org/10.1650/7329 Ewers, R. M., and R. K. Didham. 2006. Confounding factors in Knick, S. T., and J. T. Rotenberry. 1995. Landscape characteristics the detection of species responses to habitat fragmentation. of fragmented shrubsteppe habitats and breeding passerine birds. Biological Reviews 81:117-142. http://dx.doi.org/10.1017/ Conservation Biology 9:1059-1071. http://dx.doi.org/10.1046/ S1464793105006949 j.1523-1739.1995.9051041.x-i1 Fischer, J., and D. B. Lindenmayer. 2007. Landscape modification Knick, S. T., and J. T. Rotenberry. 2002. Effects of habitat and habitat fragmentation: a synthesis. Global Ecology and fragmentation on passerine birds breeding in intermountain Biogeography 16:265-280. http://dx.doi.org/10.1111/ shrubsteppe. Studies in Avian Biology 25:130-140. j.1466-8238.2007.00287.x Knight, E. C. 2013. Impacts of habitat fragmentation by Fletcher, R. J. Jr., L. Ries, J. Battin, and A. D. Chalfoun. 2007. agriculture on breeding songbirds in the Okanagan sagebrush The role of habitat area and edge in fragmented landscapes: shrubsteppe. Thesis, Simon Fraser University, Burnaby, British definitively distinct or inevitably intertwined? Canadian Journal Columbia, Canada. of Zoology 85:1017-1030. Krannitz, P. G. 2007. Abundance and diversity of shrub-steppe Foley, J. A., R. DeFries, G. P. Asner, C. Barford, G. Bonan, S. R. birds in relation to encroachment of ponderosa pine. Wilson Carpenter, F. S. Chapin, M. T. Coe, G. C. Daily, H. K. Gibbs, J. Journal of Ornithology 119:655-664. http://dx.doi.org/10.1676/06-129.1 H. Helkowski, T. Holloway, E. A. Howard, C. J. Kucharik, C. Kuehl, A. K., and W. R. Clark. 2002. Predator activity related to Monfreda, J. A. Patz, I. C. Prentice, N. Ramankutty, and P. K. landscape features in northern Iowa. Journal of Wildlife Snyder. 2005. Global consequences of land use. Science Management 66:1224-1234. http://dx.doi.org/10.2307/3802955 309:570-574. http://dx.doi.org/10.1126/science.1111772 Avian Conservation and Ecology 9(1): 8 http://www.ace-eco.org/vol9/iss1/art8/
Lahti, D. 2001. The “edge effect on nest predation” hypothesis America Number 390. Cornell Lab of Ornithology, Ithaca, New after twenty years. Biological Conservation 99:365-374. http://dx. York, USA. doi.org/10.1016/S0006-3207(00)00222-6 Sauer, J. R., J. E. Hines, J. E. Fallon, K. L. Pardieck, D. J. J. Lahti, D. C. 2009. Why we have been unable to generalize about Ziolkowski, and W. A. Link. 2014. The North American breeding bird nest predation. Animal Conservation 12:279-281. http://dx. bird survey, results and analysis 1966 - 2012, Version 02.19.2014. doi.org/10.1111/j.1469-1795.2009.00286.x U.S. Geological Survey, Patuxent Wildlife Research Center, Laurel, Maryland, USA. Leopold, A. 1933. Game management. Charles Scribner’s Sons, New York, New York, USA. Shaffer, T. L. 2004. A unified approach to analyzing nest success. Auk 121:526-540. http://dx.doi.org/10.1642/0004-8038(2004)121 Lokemoen, J. T., and R. R. Koford. 1996. Using candlers to [0526:AUATAN]2.0.CO;2 determine the incubation stage of passerine eggs. Journal of Field Ornithology 67:660-668. Sisk, T. D., and J. Battin. 2002. Habitat edges and avian ecology: geographic patterns and insights for western landscapes. Studies Mabee, T. J. 1998. A weather-resistant tracking tube for small in Avian Biology 25:30-48. mammals. Wildlife Society Bulletin 26:571-574. Smith, M. L. 2004. Edge effects on nest predators in two forested Major, R. E., and C. E. Kendal. 1996. The contribution of landscapes. Canadian Journal of Zoology 82:1943-1953. http://dx. artificial nest experiments to understanding avian reproductive doi.org/10.1139/z05-003 success: a review of methods and conclusions. Ibis 138:298-307. http://dx.doi.org/10.1111/j.1474-919X.1996.tb04342.x Spanhove, T., V. Lehouck, and L. Lens. 2009. Edge effects on avian nest predation: the quest for a conceptual framework. Mazerolle, M. J. 2013. Package 'AICcmodavg'. The R Project for Animal Conservation 12:284-286. http://dx.doi.org/10.1111/ Statistical Computing, Vienna, Austria. j.1469-1795.2009.00293.x Nur, N., S. L. Jones, G. R. Geupel, and G. Geupel. 1999. Statistical Sperry, J. H., D. A. Cimprich, R. G. Peak, and P. J. Weatherhead. guide to data analysis of avian monitoring programs. U.S. 2009. Is nest predation on two endangered bird species higher in Department of the Interior, Fish and Wildlife Service, habitats preferred by snakes? Ecoscience 16:111-118. http://dx. Washington, D.C., USA. doi.org/10.2980/16-1-3198 Paczek, S., and P. Krannitz. 2004. The importance of floristics to Stauffer, G. E., D. R. Diefenbach, M. R. Marshall, and D. W. sagebrush breeding birds of the south Okanagan and Similkameen Brauning. 2011. Nest success of grassland sparrows on reclaimed Valleys, British Columbia. U.S. Department of Agriculture Forest surface mines. Journal of Wildlife Management 75:548-557. http:// Service, Asilomar, California, USA. dx.doi.org/10.1002/jwmg.70 Paton, P. W. C. 1994. The effect of edge on avian nest success: Stephens, S. E., D. N. Koons, J. J. Rotella, and D. W. Willey. 2004. how strong is the evidence? Conservation Biology 8:17-26. http:// Effects of habitat fragmentation on avian nesting success: a review dx.doi.org/10.1046/j.1523-1739.1994.08010017.x of the evidence at multiple spatial scales. Biological Conservation Pitkin, M., and L. Quattrini. 2010. Pocket guide to sagebrush birds. 115:101-110. http://dx.doi.org/10.1016/S0006-3207(03)00098-3 Rocky Mountain Bird Observatory and PRBO Conservation Vander Haegen, W. M. 2007. Fragmention by agriculture Science, Petaluma, California, USA. influences reproductive success of birds in a shrubsteppe R Core Team. 2012. R: a language and environment for statistical landscape. Ecological Applications 17:934-947. http://dx.doi. computing. The R Project for Statistical Computing, Vienna, org/10.1890/06-0990 Austria. Vander Haegen, W. M., F. C. Dobler, and D. J. Pierce. 2000. Renfrew, R. B., and C. A. Ribic. 2003. Grassland passerine nest Shrubsteppe bird response to habitat and landscape variables in predators near pasture edges identified on videotape. Auk eastern Washington, U.S.A. Conservation Biology 14:1145-1160. 120:371-383. http://dx.doi.org/10.1642/0004-8038(2003)120[0371: http://dx.doi.org/10.1046/j.1523-1739.2000.99293.x GPNPNP]2.0.CO;2 Vander Haegen, W. M., M. A. Schroeder, and R. M. deGraaf. Ribic, C. A., R. R. Koford, J. R. Herkert, D. H. Johnson, N. D. 2002. Predation on real and artificial nests in shrubsteppe Niemuth, D. E. Naugle, K. K. Bakker, D. W. Sample, and R. B. landscapes fragmented by agriculture. Condor 104:496-506. Renfrew. 2009. Area sensitivity in North American grassland http://dx.doi.org/10.1650/0010-5422(2002)104[0496:PORAAN]2.0. birds: patterns and processes. Auk 126:233-244. http://dx.doi. CO;2 org/10.1525/auk.2009.1409 Van Horn, M. A., R. M. Gentry, and J. Faaborg. 1995. Patterns Ries, L., R. J. Fletcher Jr., J. Battin, and T. D. Sisk. 2004. of Ovenbird (Seiurus aurocapillus) pairing success in Missouri Ecological responses to habitat edges: mechanisms, models, and forest tracts. Auk 112:98-106. http://dx.doi.org/10.2307/4088770 variability explained. Annual Review of Ecology, Evolution, and Weatherhead, P. J., and G. Blouin Demers. 2004. Understanding Systematics 35:491-522. http://dx.doi.org/10.1146/annurev. avian nest predation: why ornithologists should study snakes. ecolsys.35.112202.130148 Journal of Avian Biology 35:185-190. http://dx.doi.org/10.1111/ Rotenberry, J. T., M. A. Patten, and K. L. Preston. 1999. Brewer’s j.0908-8857.2004.03336.x Sparrow (Spizella brewerii). In A. Poole, editor. Birds of North Avian Conservation and Ecology 9(1): 8 http://www.ace-eco.org/vol9/iss1/art8/
Weatherhead, P. J., G. L. F. Carfagno, J. H. Sperry, J. D. Brawn, and S. K. Robinson. 2010. Linking snake behavior to nest predation in a Midwestern bird community. Ecological Applications 20:234-241. http://dx.doi.org/10.1890/09-0059.1 Wiewel, A. S., W. R. Clark, and M. A. Sovada. 2007. Assessing small mammal abundance with track-tube indices and mark- recapture population estimates. Journal of Mammalogy 88:250-260. http://dx.doi.org/10.1644/06-MAMM-A-098R1.1 Wisdom, M., L. Suring, M. Rowland, R. Tausch, R. Miller, L. Schueck, C. W. Meinke, S. Knick, and B. Wales. 2003. A prototype regional assessment of habitats for species of conservation concern in the Great Basin ecoregion and Nevada, Version 1.1. U.S. Department of Agriculture Forest Service, Pacific Northwest Research Station, La Grande, Oregon, USA. With, K. A. 1994. The hazards of nesting near shrubs for a grassland bird, the McCown’s Longspur. The Condor 96:1009-1019. http://dx.doi.org/10.2307/1369110
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